Speaking after chairing a meeting of the Government’s emergency cobra committee this morning, Mrs May said the Government was also sending a team to look at security arrangements in the area, which attracts thousands of western tourists every year.

A total of 38 people were killed in the attack at a beach resort in Sousse on Friday, while 39 others were wounded.

The Foreign Office has updated its travel advice to warn that further terrorist attacks in Tunisia are possible and urged people to be vigilant.

Mrs May told reporters: “The figure for the number of confirmed dead remains at 15, but we should expected that that figure is going to rise.”

The news comes as Mrs May indicated British officials are struggling to identify the victims of the Tunisian terror atrocity because few of the holidaymakers were carrying anything.

Mrs May said Britons gunned down in the “appalling” massacre would not have had passports or other forms of identification with them on the beach and some victims were being moved between hospitals in the country.

Mrs May said there was no evidence that gunman Seifeddine Rezgui had chosen the Sousse resort because of the high number of Britons that flock to the popular holiday destination.

But the terror threat Britain is facing is becoming more diverse with a possibility of more spontaneous lone wolf attacks attacks, she added.

Over the last decade about 40 terror plots are believed to have been foiled by the security services.

Mrs May told BBC One’s Andrew Marr Show: “What’s important is that the information is absolutely 100% correct when it is given to a family.

​​“If you can imagine, these are circumstances, because of the circumstances of the attack, people won’t have had documents with them, they won’t necessarily have had phones.

“Some people are being moved between hospitals, so the team on the ground are making every effort to ensure that they get accurate information.”

Asked if the beach had been targeted because of the high number of British tourists using it, she replied: “This is still ongoing investigations and we are working very closely with the Tunisian authorities in relation to this.

“I’ve seen no evidence so far that this was targeted because there are British tourists there, but, of course we must recognise this is the most significant loss of life in a terrorist attacks since 7/7.”

Asked about reports that four major plots in London in the last few months have been halted, Mrs May said: “Yes, a number of plots have been stopped.

“Over the last ten years it’s estimated that something like 40 plots have been disrupted here in the UK.

“The threat has become more diverse over time. It has changed over time. That’s why it’s important for us to sustain the counter-terrorism capabilities of the agencies.

“We see that you have not just those who will plot for some time, perhaps in a group to undertake a complex attack, but also the possibility of more spontaneous attacks, these so-called lone wolf attacks.”

Mrs May said the government will be introducing a counter-extremism strategy to tackle radicalisation.

Since December 2013 more than 70,000 pieces of extremist material have been removed from the internet, she added.

Mrs May said: “We want to work with families and communities so if they are seeing signs of somebody being radicalised, if they are concerned about the behaviour perhaps of their son or daughter or perhaps of somebody’s friend that they are actually able to come forward to the authorities and talk to us about this so we can perhaps stop somebody from going down that path of radicalisation.”

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